The long term goal of this project is to understand the antiarrhythmic effects, including antifibrillatory effects, of clinical antiarrhythmic drugs on the cardiac Na channel. Understanding the actions of antiarrhythmic drugs at the cellular electrophysiological level are important because they should allow for prediction of clinical efficacy and minimize expensive clinical studies such as the recent Cardiac Arrhythmia Suppression Trial (CAST) with encainide and flecainide. Because most antiarrhythmic drugs block Na channel conductance and cause the study of drug-channel interactions to be difficult, this application proposes to initially study the interactions of key pharmacological modifiers on the cardiac Na channel. These channel modifiers permit direct measurement of modified Na channel behavior and should serve as a paradigm for drug interactions with the Na channel. Subsequent to understanding changes in Na channel gating by channel modifiers, the understanding of the actions of clinical antiarrhythmic drugs should be facilitated by the knowledge of channel interactions with modifiers on the cardiac Na channel structure- function relationship. Because the cardiac Na channel differs from that from nerve and little is known of the actions of these channel modifiers in heart, this application proposes to determine the actions of key channel modifiers on channel activation and inactivation of whole-cell Na currents (INa) and of single channel currents (iNa) and relate these changes to modified Na channel molecular conformational transitions measured by Na channel gating current (Ig). The specific aims are: 1. Characterize the actions of batrachotoxin (and other lipophilic binding site 2 toxins) on the canine cardiac Purkinje Na channel including the whole-cell current, single channel current and gating current. 2. Characterize the actions of the hydrophilic, polypeptide sea anemone II toxin (and other binding site 3 toxins) on the cardiac Na channel. 3. Remove Na channel fast inactivation with alpha-chymotrypsin and with chemical agents (N-bromoacetamide and chloramine-T) and compare their effects on membrane ionic current, gating current and single channel currents with those of binding site 2 and 3 toxins. 4. Develop a kinetic model of the modified Na channel for each toxin and chemical agent based upon their respective measurements of INa and Ig. These studies will contribute to the development of a strong foundation for the understanding of drug-Na channel interactions and will advance the long term goal of understanding the antiarrhythmic effects of clinical antiarrhythmic drugs on the cardiac Na channel.